In recent years, with development of various devices and systems, performance improvement of storage batteries as power sources has been demanded. In particular, lithium ion batteries are widely used as secondary batteries serving as power sources of electronic devices such as mobile communication devices and notebook computers. Further, for reducing the environmental load, they have been rapidly expected also as batteries for driving motors of automobiles and stationary storage batteries, and thus further improvement in capacity has been desired.
In lithium ion batteries, lithium cobalt oxide (LiCoO2) is mainly used as a positive electrode material. However, it is one of the causes of increasing the cost of lithium ion batteries due to a large content of cobalt that is a rare metal. Particularly for in-vehicle use or the like, since the size of batteries increases, use of LiCoO2 is difficult due to its high cost.
As a substitute material for lithium cobalt oxide, lithium nickel oxide (LiNiO2) and lithium manganese oxide (LiMn2O4) and the like, which are made of elements that are inexpensive and abundant as natural resources, have been used in some cases. However, LiNiO2 has a problem of reducing the battery safety during charge. Further, LiMn2O4 may cause deterioration of battery characteristics due to dissolution of divalent Mn into the electrolyte during charge and discharge at high temperature.
As means for overcoming the above-described problem, Patent Literature 1 discloses a lithium manganese composite oxide containing Ti for preventing the dissolution of Mn. Patent Literature 2 discloses a lithium ferrite composite oxide containing Fe for further reducing the cost. Patent Literature 3 discloses a method for producing a lithium ferrite composite oxide produced using a manganese compound, an iron compound, and a nickel compound. Patent Literature 4 discloses a method for coating surfaces of particles of an electrode material with carbon using an organic compound consisting of four benzene rings such as pyrene. Patent Literature 5 discloses a technique for improving the structural and thermal stability by coating the surface of a positive electrode material with a metal oxide. Patent Literature 6 discloses a carbon-coated lithium transition metal-containing compound in which not less than 85% of the surface of a lithium transition metal-containing compound is coated with a carbon material.